Increasingly, there is interest in the capabilities of nuclear reactors to transform and thereby destruct through reactor burnup large quantities of weapons grade plutonium. It is common, e.g. in boiling water reactors, to employ fissionable material, such as uranium and minor amounts of other fissionable materials, such as plutonium or thorium in the fuel pellets. Additionally, neutron absorbers are frequently used in the nuclear fuel pellets to control the inherent excess reactivity of the fuel in the core to achieve greater efficiency and economy and to prolong the service life of the fuel. For example, in boiling water reactors, uranium, which has initial excessive reactivity, is combined with a depletable neutron absorber, commonly referred to as a burnable poison, such as gadolinium. This initial excessive fuel reactivity is tempered by the introduction of the depletable neutron absorber which progressively expends its capacity to absorb neutrons. Thus, the burnable poison absorbs excess neutrons to level or stabilize the fuel reactivity rate during the period of initial excessive reactivity and then subsequently absorbs neutrons at a decreasing rate approximately commensurate with the diminishing reactivity of the fuel whereby a substantially constant rate of reactivity is maintained. In a typical boiling water reactor, the vast majority of the fuel rods of a fuel bundle comprise fissile uranium material with only a very few of the rods having a combination of the fissile uranium and a burnable poison, such as gadolinium.
While plutonium has previously been considered as an alternate fuel for boiling water reactors, as well as a combination of fissile uranium and plutonium with a burnable poison, for example, see U.S. Pat. No. 5,089,210 of common assignee herewith, it has been commonly believed that there is a severe limitation with respect to the quantity of plutonium which may be used in boiling water reactors. Particularly, it has previously been thought that no more than about one-quarter of the fuel rods in a reactor may be loaded with plutonium or mixed oxide fuel without unacceptable operational consequences. Also, reactor designers heretofore believed the presence of plutonium would interfere with the effectiveness of the gadolinium reactivity control. This is indeed the case when gadolinium is loaded into 10 to 20% of the rods, as is typical in reactors of this type. Hence, use of plutonium as a fuel in nuclear reactors has been inhibited by these and other considerations.